The conventional exposure apparatus used in fabrication of semiconductor devices and others is arranged to project a circuit pattern formed on a mask (reticle), through a projection optical system onto a photosensitive substrate (e.g., a wafer) to transfer the circuit pattern onto the photosensitive substrate. The photosensitive substrate is coated with a resist, and the projection exposure through the projection optical system results in exposing the resist to light and obtaining a resist pattern corresponding to the mask pattern. Here the resolving power W of exposure apparatus is dependent upon the wavelength λ of exposure light and the numerical aperture NA of the projection optical system and is expressed by Eq (a) below.W=k·λ/NA (k: constant)   (a)
In order to enhance the resolving power of exposure apparatus, it is thus necessary to decrease the wavelength λ of exposure light and to increase the numerical aperture NA of the projection optical system. In general, it is difficult to increase the numerical aperture NA of the projection optical system over a predetermined value in terms of optical design, and therefore it becomes necessary to further decrease the wavelength of exposure light. Techniques of EUVL (Extreme UltraViolet Lithography) are thus drawing attention as a next-generation exposure method (exposure apparatus) for semiconductor patterning.
The EUVL exposure apparatus uses EUV (Extreme UltraViolet) light in the wavelength range of approximately 5-20 nm, in comparison with the conventional exposure methods using KrF excimer laser light at the wavelength of 248 nm and ArF excimer laser light at the wavelength of 193 nm. When the EUV light is used as exposure light, there is no optically transparent material available. For this reason, the EUVL exposure apparatus necessarily uses a reflective optical integrator, a reflective mask, and a reflective projection optical system (e.g., cf. Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open No. 11-312638